In general, the problem associated with heat and power plants is the ability to effectively reduce carbon monoxide, a toxic gas, to a non-toxic level. There are three reactions that take place before the fuel cell can produce electrical energy.
The hydrogen required to operate a fuel cell unit can be obtained in a downstream gas production system by the reforming of hydrocarbons. Steam reforming of hydrocarbons to produce hydrogen is a known process. In accordance with equation (1), a reformate gas is formed which contains hydrogen and carbon monoxide. In equation (1), methane is used as an example of a hydrocarbon.Steam reforming CH4+H2O→3H2+CO  (1)
Steam reforming in accordance with equation (1) is normally performed so that optimum amount of methane is reacted. This requires high operating temperatures in the steam reformer, between 700 and 800° C.
Since the carbon monoxide contained in the reformate gas is a strong catalyst poison for the anode catalysts in fuel cells, it has to be removed as much as possible. The carbon monoxide contained in the reformate gas is therefore generally reacted with water to give hydrogen and carbon dioxide in accordance with equation (2) (the water gas shift reaction).Water gas shift reaction CO+H2O→H2+CO2  (2)
Two shift reactions are frequently used in order to react the carbon monoxide as much as possible. A high temperature shift reaction occurs at temperatures between 350 and 450° C. and a low temperature shift reaction occurs at temperatures between 180 and 250° C. Cooling the reformate gas to the relevant operating temperatures prior to entering the shift reactors is achieved with the aid of heat exchangers. The heat extracted from the reformate gas by the heat exchanger may be used to heat buildings.
However, when using shift reactions, the concentration of carbon monoxide in the reformate gas can be reduced only to the relevant equilibrium concentration at the particular reaction temperature. This is about 0.5 vol. % for the low temperature shift reaction at an operating temperature in the range 180 to 250° C. This concentration of carbon monoxide is too high and has to be reduced further, if possible to less than 100 vol.ppm or even better is less than 50 vol.ppm. The selective oxidation of carbon monoxide to carbon dioxide on suitable catalysts is usually used for this purpose. Selective oxidation is also called preferential oxidation, abbreviated to PROX. The problem with PROX is the possibility that, due to insufficient selectivity, a certain fraction of the hydrogen is also oxidized to water and thus is no longer available as a fuel gas for the fuel cell. To solve this problem, there is an alternative possibility of reacting the carbon monoxide with the hydrogen in the reformate gas in accordance with equation (3) to give methane (methanisation). The methanisation of carbon monoxide is inherently associated with the consumption of hydrogen unlike preferential oxidation.Methanisation 3H2+CO→CH4+H2O  (3)
Fuel cells with integrated gas production systems are used for supplying buildings with electrical power and heat. In the context of this invention, such a system is called a combined heat and power plant.
The hydrogen contained in the fuel gas for the fuel cell is not fully reacted at the anode of the fuel cell. Approximately 20 vol. % of the hydrogen supplied to the fuel cell leaves the fuel cell unit with the anode waste gas. The most efficient use of the primary energy carrier used, such as for example natural gas, the un-reacted hydrogen in the anode waste gas from the fuel cell is usually burnt in a gas burner to heat the steam reformer to its operating temperature.
For metering the amounts of gas required to operate the gas burners or steam reformers, gas controllers are used that measure and control the gas flow in accordance with the predetermined conditions.
Gas burners are subject to stringent safety regulations. Such as the gas burner must ensure rapid emergency shut-down of the fuel gas supply if the burner flame is extinguished, by monitoring the burner flame. Ionisation detectors are used to monitor the burner gas flame but, for problem-free operation, these require a minimum proportion of hydrocarbons in the fuel being supplied. When operating a burner with pure hydrogen, not enough ions are produced in the flame for the detector to operate effectively. Combustion of the anode waste gas in the gas burner of the steam reformer in units of the type described above which are known from the prior art therefore provides considerable problems when monitoring the burner.
Based on the forgoing there is a need in the art for a combined heat and power plant and a process that can be more cost effective and substantially simpler than those plants known in the prior art. There is also a need for a combined heat and power plant that can be operated in such a way that the safety regulations mentioned above can be observed.